Old magistrate constantly returning to drink last of wine (5)
I believe the answer is:
reeve
'old magistrate' is the definition.
(I've seen this before)
'constantly returning to drink last of wine' is the wordplay.
'constantly' becomes 'ever' (I've seen this before).
'returning' says the letters should be written backwards.
'to drink' is an insertion indicator.
'last of' indicates one should take the final letters.
The final letter of 'wine' is 'e'.
'ever' reversed gives 'reve'.
'reve' enclosing 'e' is 'REEVE'.
(Other definitions for reeve that I've seen before include "Anglo-Saxon official" , "thread his way through" , "magistrate of long ago" , "Pass the end of a rope through the bailiff" , "The bailiff to run a rope through" .)